1. Field of the Invention
The present invention relates to a method of machining nozzle holes in a nozzle body composing a fuel injection nozzle for internal combustion engines, an apparatus for machining the nozzle holes, and fuel injection nozzles produced using the method and apparatus.
2. Description of the Related Art
A fuel injection nozzle has been widely used which is composed such that a needle valve is placed for reciprocation in a central hollow of a nozzle body having a plurality of injection holes, and fuel is allowed to be injected through the injection holes provided in the downstream side of the seating position of the needle valve intermittently by allowing the needle valve to be seated on or departs from the seat face. In recent years injection nozzles have been required to improve in fuel atomization in point of view of reduction in fuel consumption, improvement in exhaust gas emission, stability in operation of internal combustion engines.
To improve atomization of injected fuel, it is important to introduce fuel with reduced loss of energy given to fuel to the injection holes and increase velocity of fuel injected through the injection holes, that is, to increase flow rate per injection hole area and per injection pressure, and it is known that rounding of the entrance corners of the injection holes is effective. Rounding of the entrance corners of the injection holes of the nozzle body by flowing abrasive fluid containing abrasive grains through the holes in order to reduce entrance resistance of fuel to the holes has been widely adopted.
One of important problems to be solved in processing of rounding the entrance corners is to even fuel injection characteristic of each of the injection holes. Injection holes are formed by drilling or laser processing beforehand as crude processing in the nozzle body. However, it is usual that there are variations in diameter and burs remaining at the entrance of each of the crude processed holes. And there has been problems that such variations can not be eliminated by abrasive fluid flowing processing and fuel injection characteristic of each injection hole is not evened. When fuel injection characteristic of each of the injection hole is not even, local high temperature zones and fuel rich zones occur in the combustion chamber of the engine resulting in decreased combustion efficiency and deteriorated exhaust emission.
As means to solve such problems, methods of controlling timing of stopping abrasive fluid flowing processing to obtain nozzle bodies having even fuel injection characteristic are discloses in Japanese Laid-Open Patent Application No. 7-52022 (patent literature 1) and Japanese Laid-Open Patent Application No. 9-209876 (patent literature 1). Further, a method of inserting a flow rectifying pipe into the nozzle body to reduce stagnation zone area of abrasive fluid in the forefront space in the nozzle body to a minimum in abrasive fluid flowing processing and make rounding of the entrance corners of the injection holes even.
However, according to the methods disclosed in the patent literature 1 and 2, pressurized abrasive fluid is supplied into the central hollow of the nozzle body without anything inserted into the central hollow. In actual operation of engines, a needle valve is inserted into the central hollow of the injection body, and fuel injection is controlled by allowing the needle valve to be seated on or departed from the seat face in the central hollow of the injection valve. Therefore, flow condition of the abrasive fluid in abrasive fluid flow processing differs largely from actual flow condition of fuel when fuel is injected. As a result, unexpected separation of fuel flow may occur near the needle valve and injection holes in actual operation of engines, occurrence of cavitation erosion is induced, resulting in occurrence of breakage failure in the injection nozzle and uneven fuel injection characteristic.
According to the method disclosed in the patent literature 3, a flow rectifying pipe is inserted into the central hollow in the nozzle body so that the opening at the nose of the rectifying pipe is positioned downstream of the injection holes, and abrasive fluid is flowed between the outer wall of the flow rectifying pipe and the inner wall of the central hollow of the injector body. By this, stagnation zone area of abrasive fluid in the forefront space in the nozzle body and even rounding around the entrance corner of each of the injection holes is realized. However, the flow passage of abrasive fluid is different from the actual flow passage of fuel when fuel is injected in actual operation of engines.
Therefore, flow condition of abrasive fluid in abrasive fluid flow processing is different from actual flow condition of fuel when fuel is injected as is in the methods of the patent literature 1 and 2. As a result, unexpected separation of fuel flow may occur near the needle valve and injection holes, occurrence of cavitation erosion is induced, resulting in occurrence of breakage failure in the injection nozzle and uneven fuel injection characteristic.
Further, the purpose of making injection characteristic of each injection hole even is not attained enough by the methods disclosed in the patent literatures in which timing of stopping abrasion fluid flowing processing is controlled is determined by detecting timing to stop processing.
The patent literature 1 discloses a method in which a flow control device is provided to the outlet side of each of the injection holes and abrasive fluid flowing processing for any one of the injection holes is stopped when flow rate of abrasive fluid through said one hole reaches a predetermined value.
Generally, fuel injection quantity per injection hole (abrasive fluid per injection hole) Q is determined by the flow coefficient μ of injection hole inlet, injection hole area A, pressure difference □P between injection hole inlet and outlet, and further by opening period of the injection hole (abrasive fluid flowing processing period) t and given by the following equation (1).Q·∝μAt□P1/2  (1)
Pressure difference □P is controlled to be equal for each injection hole. Therefore, to control timing of stopping processing for every injection hole independently so that quantity of abrasive fluid flowed through each injection hole is constant means to control including time (to control so that μA is constant), and μA of an injection hole through which abrasive fluid flowed for a longer time period until flow quantity reaches a determined value is different from that of an injection hole through which abrasive fluid flowed for a shorter time period until flow quantity reaches a determined value. In actual operation of engines, electromagnetic valves control injection time period of each injection nozzle to control engine operation, so a period of time that pressure exerts on each injection hole in a cycle is constant. Therefore, variation in μA induces variation in fuel injection quantity and spray characteristic (atomized fuel particle diameter, spray distribution, etc.)
In the method disclosed in the patent literature 2, the flow meter is located upstream of the nozzle body and flow rate of abrasive fluid flowing through all injection holes of the nozzle body is measured. The processing is stopped when the flow rate reaches a predetermined value. With this method, as flow rate through each individual injection hole can not be controlled, variations in fuel injection characteristic may remain in individual injection holes due to variations in surface roughness and burrs around individual injection holes.
The injection hole machining method disclosed in the patent literature 3 consists of a first step and second step of processing for the purpose of eliminating influence of variation in diameter and surface roughness of injection holes before performing abrasive fluid processing, in the first step abrasive fluid flowing processing being performed under low pressure to even the diameter of each injection hole, and in the second step abrasive fluid flowing processing being performed under higher pressure to round the entrance corner of each of the injection hole. In the first step, diameter of each injection hole is estimated based on measurement result of flow rate of abrasive fluid through each injection hole, and abrasive fluid flow rate is estimated for each injection hole and controlled to obtain target diameter of injection holes. In the second step, abrasive fluid is flowed at the same flow rate for all of the injection holes to round entrance corners of the injection holes. However, with the method, only variation in diameter of injection holes is taken into consideration, variation in surface roughness and small and large of burrs near entrances of injection holes. Therefore, even if variation in diameter of injection holes is eliminated by the processing, there may remain variation in rounding of entrance corners even after the second step of the processing. Further, processing time increases, since the processing is divided in two steps.